to answers, "SOlid is the way to go" and "it's probably not the best choice" what are your reasons. As far as experience I have a lot of experience with fiberglass (west system) over the years (boat and car repair, canoe construction built a 16' glass over wood boat years ago etc.) I also have a lot of experience with wood (finishing carpenter, fine furniture and Luthier). one problem with wood is If I am going to use Balsa or Coosa I'll have to order it in where as I have everything I need for west system

It would help if you quoted your intended target... I had to go searching as to whom you were asking .

IMHO... The only reason it was not soil to begin with in your and my boats is the builder was to cheep to do it..A lot of top of the line boats have a sold core where intended deck penetrations are..cheaper built bots not so much.

You ask why...Simply water intrusion ....intrusion and compromise of the core material rather wood or foam that you are currently facing and would not and will not ever be a future issue with solid glass core in that area.
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The only reason for any plywood or balsa core in the first place was for a stiffner..and cheap stiffening at that...There is an exception when an insulating factor is desired in the hulls construction..but a modern sandwich is ultra light and ultra strong..but still generally solid glass where penetrations are engineered to go through either their decks or hull.

I have no clue as to why it was suggested as a bad idea to go solid in your mast step area..maybe he will respond to his answer.

FWIW...repairing it from the top will be WAY easier..just way harder to get your top gel-coat looking factory nice that's all..as you will have gravity working for you not against you...as I said earlier practice with 2 or three goes at mixing and saturating some scrap pieces together to get the hang of saturating and working with mat and resin..it ain't rocket science..learn to work all the air and excess resin out of the mat..gobbing on way to much resin is a beginners fist mistake..you dont want any unsaturated areas but you dont want it swimming in the stuff either..it will be brittle and crack.

You say you have lots of experiance with glass...Then Go For it!..If I was there I would help you.

__________________"Go Simple...Go Large"

Relationships are everything to me..everything else in life are just tools to enhance them.

The purchase price of a boat is just the admittance fee to the dance...you still have to spend money on the girl...so court one with something going for her with pleasing and desirable character traits others desire as well... or you could find yourself in a disillusioned relationship contemplating an expensive divorce.